This invention relates to discharge devices and more particularly to a high pressure, mercury-vapor, discharge lamp of the additive type wherein an additive metal iodide is utilized to supplement the usual mercury discharge.
High pressure, mercury vapor, discharge lamps (HPMV lamps) are well known and have been used extensively for highway and industrial lighting purposes, as well as numerous other applications. Although the color emitted by the mercury lamp is not entirely displeasing, the color rendition of illuminated objects leaves something to be desired in that the visible spectral radiations emitted are concentrated primarily in the green and yellow. In the non-additive mercury vapor lamp there is a distinct lack of radiation in the blue green region and also in the orange and red spectral regions. Although these deficiencies have been corrected to a degree by phosphor coating of the outer bulb, problems in increased source size and the efficient utilization of light are thereby introduced.
Considerable effort has been expended to improve the color rendition provided by high pressure vapor arc lamps of the HPMV type. Particular emphasis has been placed upon the substitution of various metal vapors for mercury and on the use of metal and/or metal iodide additives in conventional mercury-vapor lamps. Although color rendition has in many cases been improved by these efforts other problems of even greater moment have been introduced. For example, many of the metal substitutes such as cadmium and zinc are difficult to vaporize, and sodium metal, which provides comparatively good color rendition at high temperatures, is less difficult to vaporize but cannot be used with quartz of temperatures above 300.degree. C due to a severe reaction with the quartz. Many of the additive type lamps introduce still different problems, for example, some iodides, such as sodium iodide, have a tendency to react with the arc tube and possibly with other arc tube parts during operation. This reaction releases free iodine which in turn increases the voltage required to initiate the discharge and attacks other parts within the arc tube which reduces the lamp life considerably.